The present invention generally relates to demodulating received signals, and particularly relates to demodulating signals having potentially unknown nominal amplitudes, such as in 16-ary Quadrature Amplitude Modulation (16QAM).
Evolving wireless communication standards look to a range of techniques to increase communication data rates. Modulation format, modulation and coding rate adaptation, incremental redundancy, hybrid ARQ, and fast scheduling all are coming into increasing use in the effort to increase data rates while maintaining acceptable robustness.
In particular, the adoption of relatively complex amplitude modulation formats poses significant challenges with regard to reliable signal demodulation. For example, quadrature amplitude modulation (QAM) schemes define “constellations” of symbol points, with each point having a unique combination of phase and amplitude. Commonly, the constellation points are symmetrically arranged around a zero amplitude origin, such that each phase quadrant has a like number of similarly positioned constellation points. 16QAM defines sixteen constellation points, with four points in each phase quadrant. Notably, the relative positioning of points within the constellation is fixed, but the absolute amplitudes (i.e., the distance from the constellation's amplitude origin) of the constellation points vary as a function of received amplitude, for example.
Commonly, particularly in CDMA systems, the remote receiver receives a reference signal (or signals) in conjunction with one or more data signals, e.g., traffic and/or control channel signals. The reference signal may be, for example, a pilot channel signal, pilot symbols, etc. In any case, the receiver knows the nominal reference signal and thus it uses the actual reference signal as received to estimate radio channel properties for the associated received data signal(s). Thus, the receiver maintains “channel estimates” based on receiving the reference signal, wherein the channel estimates characterize the current phase and attenuation properties of the radio channel. Thus, the receiver compensates the received data signal for channel effects using these channel estimates to thereby improve its reception performance.
However, where the received amplitudes of the reference and data signals is different, such as commonly occurs where the signals are transmitted at different transmit powers, the reference signal does not directly indicate what the absolute amplitudes of the received data signal symbols (constellation points) should be. Thus, the receiver may encounter difficulty in deciding which constellation points have been received.
Therefore, it is known to transmit additional information, i.e., data signal amplitude information, to facilitate demodulation of the received data signal by the remote receiver. Alternatively, it is known to transmit the data signal(s) at fixed relative amplitudes such that the receiver knows a priori the amplitude offset between the reference and data signals.